1. Field of the Invention
This invention relates to an optical pickup head in an optical disc drive, and more particularly to a central servo controller (CSC) used to reduce fluctuation of the optical pickup head in the optical disc drive through a digital signal processing technology.
2. Description of Related Art
A basic structure for an optical disc drive is shown in FIG. 1. In FIG. 1, an optical disc drive 100 includes an optical pickup head (PUH), a rotating disc 110, which is rotated by a spindle motor 120 or a disc motor. The PUH is held by a sled motor 130 and a tracking coil 140. A radio-frequency (RF) amplifier 150, a motor driver 160, and a digital signal processing (DSP) chip 170 are further used to control the sled motor 130 and the tracking coil 140. The DSP chip 170 includes a central servo controller (CSC) 172.
Generally, a seeking action is first needed to read data stored on the disc 110. The seeking action includes locating the optical pickup head PUH to a desired track. The optical pickup head PUH is driven by the sled motor 130 and the tracking coil 140. Then, the rotating motor 120 rotates the disc 110. The optical pickup head PUH read data stored on the disc 110 through optical technology. The read data and related information responded by the optical pickup head PUH are converted into electrical signals and amplified by the RF amplifier 150. Output information of the RF amplifier 150 at least includes RF data signals, a tracking error signal (Te), and a central servo signal (Tcs), all of which are sent to the DSP chip 170. The central servo controller CSC 172 produces a control signal FMO and a control signal TRO according to the sending-in signals. The control signal FMO is used to control the sled motor 130 and the control signal TRO is used to control the tracking coil 140 so as to locate a desired track position. The motor driver 160 receives the control signals FMO and TRO and properly drive the sled motor 130 and the tracking coil 140 to locate the optical pickup head PUH onto the desired track position.
The optical pickup head PulH includes a laser beam, which is incident onto the disc 110 and is reflected by the disc 110 into three reflected beams. FIG. 2 is a drawing, schematically illustrating reflected beams from the disc as the laser beam is incident onto the disc. In FIG. 2, the laser beam is vertically incident onto the disc 110, which has stored disc data. Due to diffraction effect, the laser beam is reflected to from a reflection pattern which includes three maximums: -1 order maximum, 0 order maximum, and +1 order maximum with respect to a -1st order reflected beam, 0th order reflected beam, and +1st order reflected beam. The reflected beams are usually detected by photodetectors as shown in FIG. 3, which is a drawing projected on a disc plane X-Y, schematically illustration a photodetector arrangement to detect reflected beams. In FIG. 3, there are three photodetectors 310, 320, and 330, which are respectively located on regions 310', 320' and 330'. Each of the photodetectors 310, 320, and 320 has a separating distance of 1/2 track pitch along the X-direction as denoted in FIG. 3. The photodetector 320 is located at the middle and further includes four detecting regions A, B, C, and D, the photodetector 310 includes a detection region F, and the photodetector 330 includes a detecting region E, in which each of the detecting regions includes a photosensor.
In FIG. 2 and FIG. 3, the sensors A and B of the photodetector 320 are used to detect the reflected beam in the +X-direction and the sensors C and D of the photodetector 320 are used to detect the reflected beam in the -X-direction. The central servo signal Tcs is defined as a computed quantity of (A+B)-(C+D). The central servo signal Tcs is used by the central servo controller 172 of FIG. 1 to drive the optical pickup head PUH as to be described later in detail.
FIG. 4A is a drawing projected on a disc plane, schematically illustrating a relative location of the middle photodetector on a track with two adjacent tracks. In FIG. 4A, a track on a track position X0 on the optical disc. An adjacent track on a track position X4 is beside the track position X0 at the outer side, and an adjacent track at a track position X4' is beside the track position X0 at the inner side. The inner direction points to a center point of the optical disc. The track pitch between the track position X0 and the track position X4 is divided in four equal regions as sequentially denoted by position X1, X2, and X3. Similarly, positions X1', X2', and X3' divide the track pitch between position X0 and the position X4' into four equal regions. They are substantially symmetrical.
FIG. 4 is a light intensity distribution, schematically illustrating intensity of reflected beams on the middle photodetector of FIG. 4A at different location. In FIG. 4A, when reflected beams are detected by the photodetector 320 through the four sensors A, B, C, and D. A detected light intensity distribution of the photodetector 320 is different as the location of the photodetector 320 is different. In FIG. 4B, each ellipse respectively corresponds to one of the positions X4'-X4. White region represents high intensity, light gray region represents middle intensity, and the dark gray region represents low intensity. In each ellipse, a central circle area is mainly contributed by the 0.sup.th order reflected beam, and two side half-circle areas at both side are respectively contributed by the -1.sup.st and +1.sup.st order reflected beams, which are respectively detected by the sensors C, D and the sensors A, B.
The central servo signal Tcs is obtained by the result of (A+B)-(C+D). As the photodetector 320 is at the position X0, the Tcs is zero due to symmetrical property. As the photodetector 320 shifts to the position X1 (X1') or the position X3 (X3'), the Tcs is strong due to imbalance from the +1.sup.st order and the -1.sup.st order reflected beams. The Tcs is also zero at the position X2 (X2'), which is at half of the track pitch. The Tcs distribution with sign along the track radial direction is shown in FIG. 4C, in which the sign also distinguishes the contribution from the sensors A, B or the sensors C, D. The distribution of the Tcs is like a sine periodic function. Each time of track crossing action induces one period of the sine function. The frequency of the Tcs can be converted into a track crossing velocity of the optical pickup head PUH of FIG. 1. The optical pickup head PUH is driven by the tracking coil 140.
For the typical optical disk drive shown in FIG. 1, as the sled motor 130 seeks a desire track, it typically includes an acceleration velocity and a deceleration velocity, which usually induce a position fluctuation on the tracking coil 140. If the position fluctuation severely occurs, the tracking coil 140 may result in a focus failure. Moreover, as the operation status enters a following track mode, the position fluctuation is also harmful to the read data from the optical disc 100.
FIG. 5A and FIG. 5B are drawings, schematically illustrating a relative location between a lens of the optical pickup head and the middle photodetector, when the lens is on the desired position or deviates from the desired position. In FIG. 5A, a lens 510 is located at a center of the sensors A-D without deviation. In FIG. 5B, the lens 520 deviates a little from an original area 510'. As the situation in FIG. 5B, a larger portion of the lens 520 is detected by the sensors A and B. In this manner, the intensity quantity of A+B is larger than the intensity of C+D. The Tcs obtains a positive quantity. Otherwise, the TCs is zero in FIG. 5A. The lens 520 can be controlled to stay on the central position of the photodetector 320 through the feedback signal of the Tcs. The position fluctuation issue can therefore be solved. The lens control usually is done by an external band pass filter (BPF) circuit. For the conventional manner, the Tcs is only used to locate the optical pickup head onto the desired position.
However, for various different types of optical disc drives, suitable frequency range and signal gain of the BPF circuit are necessary to be properly adjusted, resulting in inconvenient operation. Moreover, the BPF needs operational amplifiers, switching chip set, capacitors, resistors, and so on. Fabrication cost is increased.